Frozen PC unusable

I cannot use my PC. It is booting OK but nothing can be run. There is an endlessly rotating busy circle displayed whenever I click an app or other file. I have seen this before but it has ended after a short delay, this time it is unending. None of the usual fixes I have such, as Super F4, Kill It All or Explorer Restart are working. I have to close down by using power off. But on reboot it is still frozen. When it happened befor I assumed it was due to insufficient RAM being available but it happens now even when RAM usage is reported as between 50% and 70%, the usual working level.

What is causing this and what can I do about it? I am having to send this from my Android Tablet.

Thanks. After entering Safe Mode I did not know what to do. I did not find Startup Repair. I ran as many things as I could and all seemed to work well. On rebooting most of the trouble I was having had gone, I can use the PC for most of what I want, though there are some glitches with such things as programs suddenly freezing or crashing. Apart from one persistent issue with Bluetooth I will wait now until the May Update arrives, in the hope it clears up the remaining issues as it has done in the past --- unlike some other users I look forward to updates, for me they often solve problems rather than create them.

So thanks again for the reminder about Safe Mode, it has cured the immediate problem.