I was checking my email on my phone, and for some reason it glitched out and marked every single of my thousand emails unread. Is there any way I can mark all emails as read as once instead of doing 50 or 100 at a time?
If you go to the "All Mail" view from a computer (not sure how the mobile version works) and click the 'select all' checkbox, a little notification saying something like "All 50 conversations on this page are selected. Select all 24,037 conversations in All Mail" should appear, where the last part is a link. If you click that link, then click 'Mark as Read,' it should take care of the problem.
This should be done in a desktop browser (not mobile)
Enter the following in the search bar at the top of Gmail page "is:unread"
Click "select all" (checkbox above the list of messages in upper left)
Click the hyperlink in the text that pops up in the "All 100 conversations on this page are selected. Select all 56,315 conversations in All Mail"
Click the "Mark As Read" button on the top.