Your question is a particular case Ex 3.32 in Brezis's book of Functional Analysis. In fact, consider the projection of $0\in E$ into $C$.
Let $(E, |\cdot|)$ be a uniformly convex Banach space and $C \subset E$ a nonempty.
- Prove that for every $x \in E$,
$$
\inf _{y \in C}|x-y|
$$
is achieved by some unique point in $C$, denoted by $P x$.
- Prove that every minimizing sequence $\left(y_{n}\right)$ in $C$ converges strongly to $P x$.
- Prove that the map $x \mapsto P x$ is continuous from $E$ strong into $E$ strong.
- More precisely, prove that $P$ is uniformly continuous on bounded subsets of $E$.
Let's ignore the ball condition at first, and just look for a complete metric space and a nested sequence of nonempty closed sets with empty intersection. This is clearly impossible for a finite metric space, so let's try the simplest possible infinite metric space: a countable set, which might as well be $\mathbb{N}$, with the discrete metric $d(m,n)=1 \Leftrightarrow m \neq n$. Every set in this space is closed, so it's easy to find the sequence we want: for example, we can take $A_n$ to be $\{ n, n+1, n+2, \ldots\}$.
This is all very well, but of course these sets aren't actually closed balls yet. This doesn't mean we should give up on our example, though. Rather, we try to tweak the metric to make each $A_n$ into a closed ball. We also need our metric to stay complete, and the simplest way to ensure this is to keep the condition that all nonzero distances are $\ge 1$.
A ball is defined by two things: a centre and a radius. The simplest choice for centre is to have $A_n$ centred at $n$. The radii will have to be decreasing (since we want $n+1 \in A_n$ but not $n \in A_{n+1}$) as well as being at least $1$, so let's try to make the radius of $A_n$ be $1+\frac1n$.
So for $m<n$, we want $d(m,n)\le 1+\frac1m$ (so that $n$ is in $A_m$) but $d(m,n)>1+\frac1n$ (so that $m$ is not in $A_n$). The simplest way to do this is to just let $d(m,n)=1+\frac1m$.
All the above is just a way of motivating the following example: let $X$ be the space $\mathbb N$ with metric $d(m,n)=1+\frac{1}{\min (m,n)}$ for $m \neq n$. It's not hard to check that this is indeed a complete metric space, and the closed ball with centre $n$ and radius $1+\frac1n$ is $\{n, n+1, n+2, \ldots\}$.
Best Answer
Let $E_n$, $n\ge 1$, be these nonempty closed convex sets. We want to pick a point $x_n$ in each set so that they form a Cauchy sequence. If the diameter of $E_n$ tends to zero as $n\to\infty$, the Cauchy property comes automatically. Otherwise, we have to make some intelligent choices of $x_n$. I present two versions; the second, pointed out by Daniel Fischer, is more slick.
Close to the "center of the set"
One idea is to pick $x_n$ close to the "center" of each set. To make this precise, let $$r_n = \inf\{r>0: \exists x\in E_n \text{ such that } E_n\subset B(x,r)\}\tag1$$ Here $B(x,r)$ is closed ball of radius $r$ centered at $x$. (The number $r_n$ is sometimes called the Chebyshev radius of $E_n$.) Observe that $r_n$ is a decreasing sequence of positive numbers, so it has a limit, $r_n\to r_*$.
As usual in infinite dimensions, we don't know if the infimum (1) is actually attained. So we must provide some slack: pick $x_n\in E_n$ such that $E_n\subset B(x_n, r_n+2^{-n})$.
I claim that the sequence $(x_n)$ is Cauchy. Indeed, suppose there is $\epsilon>0$ such that there are arbitrarily large indices $n<m$ for which $\|x_n-x_{m}\|\ge \epsilon$. Then $$E_m \subset B(x_n,r_n+2^{-n})\cap B(x_m,r_m+2^{-m})$$ Observe that both radii here can be made arbitrarily close to $r_*$ by choosing $n,m$ large. Using the parallelogram law, you can show that $$B(x_n,r_n+2^{-n})\cap B(x_m,r_m+2^{-m})\subset B\left(\frac12 (x_n+x_m),\rho\right)$$ for $\rho<r_*$, thus arriving at a contradiction.
(Draw a picture of two intersecting balls of nearly the same radius: if the distance between their radii is substantial, the intersection is contained in a ball of smaller radius).
Close to the origin of the space
Another idea is to pick $x_n$ of small norm. Let $$R_n = \inf\{\|x\| : x\in E_n \}\tag2$$ Observe that $R_n$ is an increasing sequence of positive numbers, so it has a limit, $R_n\to R_*$.
Pick $x_n\in E_n$ such that $\|x_n\| < R_n+2^{-n}$.
I claim that the sequence $(x_n)$ is Cauchy. Indeed, suppose there is $\epsilon>0$ such that there are arbitrarily large indices $n<m$ for which $\|x_n-x_{m}\|\ge \epsilon$. Then $$\frac{x_n+x_m}{2}\in E_m$$ by convexity. Using the parallelogram law, you can show that $$\left\|\frac{x_n+x_m}{2}\right\| <R$$ when $m,n$ are sufficiently large, thus arriving at a contradiction.