Reboot your computer

I get it often, even on a fresh reboot with only PS running, happens more often if Ive got multiple years of PS open at the same time, but sometimes only the current year is open, so Ive got no magic fix for this one.    Brand new system with tons of HD space and double the recommended RAM, so I know its not my system, its crappy programming with PS.

You just have to wait it out.

@IRonMaN @Just-Lisa-Now- 

Thanks, I did reboot still I have it.

One client or all clients?

I get it just sitting in Homebase, not even trying to access a client file.

On Homebase for all before picking up a client.

Could be your anti-virus not accepting this program all of a sudden.

Could be your Cache is not cleared and too much is in open RAM. To do the latter go to File Explorer, then to This PC. Then to Local disk (C:) <-- if that's your hard drive, Right Click on Local Disk (C:) and click on Properties. Click on Disk Cleanup and check every box in the list. Finally click Ok.  It may work immediately if this is your problem, or it may take one more reboot.

You should turn your computer off not just restart

shut down and reopen, it happens once in a while.