As others have pointed out, they're unlikely to give you reimbursement, even if the issue was mechanical. Perhaps if you were Global Services they would but for 99% of travellers the answer is too bad so sad! What you could do is cancel out your UA reservation which I believe they are required to give a full refund in those instances and rebook on another airline with availability. Better yet, ask UA if they can push you on to an interline partner's flight like AA. You could also check with UA to see if they've got any availability out of their co-terminal, EWR. I suspect the answer is yes given how often UA flies out of EWR, their hub, and would be surprised if UA had any qualms with you switching co-terminals like that (fellow FTers please correct me if I'm wrong here). As others have noted train/bus/car rental are other options when it becomes really severe and in some cases it may make sense to go up/down the Eastern seaboard (i.e. to IAD) on Amtrak to catch an airport that has normal operations.

Safe Travels,

James