Do you have an unfiltered view or test view in your Google Analytics account? If you do, start looking in that view to see if you are collecting data there. If daa is visible there it could be a filter in your main view blocking your sessions.
If you don't have an unfiltered view, add one right away with no filters or anything. Just let raw data flow into that view. The check that view after 24h to see if you can notice any difference. If the result is the same, you can most likely rule out filter issues.
Check your tracking code. Do you still have tracking code on all your pages? Does it have the correct GA ID? Look like this: UA-12345678-9

If you are using Google Chrome browser you can install the extension called Google Tag Assistant. Use it to verify that your tracking code is working and what ID it sends your data to.