When I try to open CCleaner, the CCleaner screen flashes for just a second, and then it disappears. I uninstalled and reinstalled - same thing. So it's not usable right now. Suggestions? Thanks.
That is usually a sign that an AntiVirus is blocking CCleaner from running.
What AV are you using? (and what version is it?)
What version of CCleaner?
What version of Windows?
If you are using a 3rd party AV (ie. not Windows Defender) then make sure that your AV version is up to date.
Check on their website, certain AV's are known to say that they are 'Up to date' when in fact they are an old version and it's only the definitions for that old version that are up to date.