my apple mail crashed and I can no longer open app

First I would try restarting the computer in the Safe Mode. Hold down the Shift key on restart. Be patient as the restart can take up to ten minutes while the operating system is repairing itself and cleaning its caches. Then see if the app opens. If so, restart normally and check again. If all this fails, you will need to reinstall Mail. To do that you have to reinstall your OS because Mail is part of it.

In addition to Ronasara's comments, try pressing and holding a Shift key while opening Mail from Applications or the Dock.

That action will open Mail without loading the previously selected (and potentially corrupted) email message that could be causing Mail to crash.

If that works and Mail opens, you might try rebuilding one or more Mailboxes according to Rebuild mailboxes in Mail on Mac - Apple Support.

If that works but Mail crashes upon selecting a message, you found at least one corrupted message.