Medication stop working?

Hi. Has anyone had a situation where their medication just suddenly stopped working or helping? I've been to a few headache doctors and am in the middle of finding a new one. I've tried several pain meds, with no relief. So until I find a new dr and meds, the only thing I have is Excedrin with Ibuprofen. It has always taken the edge off my headaches (unless I am having a mind blowing one), but this last headache I have now, it hasn't even done that ! I'm a little worried, considering that was my only defense against some relief. Has anyone else had that happen to them? And if so, did the meds start helping again the next time they needed them. Thanks !