Lately, When I Log Into My Gmail, All Of My Mail Has Disappeared. Help!

This sounds like an over active spam filter.  Are the emails showing up in the spam folder?  The other thing it sounds like is that the email is being downloaded.  Are you the only one with access to the account?  How secure is your password?

Gmail accounts have been hacked in the past so it is possibl;e someone has compromised the account and is grabbing your email.

I would change the password ASAP and make it secure.  8 characters or more.  Mixed numbers, letter, capitalization, and punctuation.

I would also consider not using the web interface.  Use something Like Mozilla Thunderbird.  That's what I do.  During setup you can tell it that you want to use a Gmail account and it will set everything up for you.  Then the email is all on your PC, not Gmail's servers (after download anyway).  Thunderbird will check for new mail every 5 minutes by default and download it for you.  It can either leave a copy on Gmail or delete Gmails copy.  Also this way you can carry it all with you if you use a laptop.

Other email programs do the same sort of thing, Outlook, Eudora, etc.

Gmail did have issues recently where email was vanishing but to my knowledge those were issues addressed and corrected.

Hope that helps.