First check whether a wipe has happened and that you are on the correct Steam account. If the loss is unexpected, send support your UID, player name and the approximate time.
1. Was it the weekly wipe?
The Federal Raid clears all plots, bases, weapons and cash every seven days. If those are what disappeared, it was most likely the wipe.
What carries over: cosmetics, player name, tutorial progress, settings and accumulated jail time.
2. Are you on the right Steam account?
Make sure you are logged into the Steam account you usually play on.