There have been problems with the servers over time. The error you're seeing indicates one of two issues - either your code won't compile, or the server is timing out (sometimes from excessive load). If it compiles cleanly locally on your machine AND you can get a clean check50 run from your computer, then it is usually server load.
You should resubmit the entire pset for regrading. (It needs to be submitted as a complete package.) It will recheck everything and grade it all. There are no penalties for resubmission and it will always take the grade for the latest submission.
If this answers your question, please click on the check to accept. If you found a different solution, please submit your own answer and accept it. Let's keep up on forum housekeeping. ;-)