^0 is just a representation of ASCII 0 - if you do a
man ascii you'll see the right column line up with the left values, an easy way to recall control characters. Unfortunately 0 is often used for null padding attack payloads (assuming you've ruled out a filesystem problem already).
The most important thing is what was prepending that junk - was it a script URL, a static resource, a module, etc? Somebody is either trying to exploit your system or fuzzing random systems looking for exploits.
That you got a crash means that something was successful - at least in causing the crash. It's a DoS at least, and often a crash (especially when they're trying to overrun buffers like that) can lead to a stack smash and running exploit code. That's bad, but it's not certain you were exploited (like others said, check for rootkits). If it's a certain repeating pattern you can use an iptables string match to prevent it from getting to Apache, but if your apache is up to date then you should report the exploit to Apache security too.