Mail app disappeared

Boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

If Mail still crashes after rebooting back into normal mode download and apply the macOS Mojave 10.14.6 Combo Update and try Mail again.

If still crashing download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.

Hoping to learn Said:

“Hoping to learn User level: Thanks for the tip. I was able to recover the app, but eventually it crashed again.”

———-

You are welcome.

Do you have a Time Machine Backup Prior to All of this?

• If yes, then Restore from the Time Machine Backup
• If not, preform as instructed here: How to Reinstall the macOS Anew - No TMB Prior to Upgrade - Apple User Tips

Hoping to learn Said:

"Mail app disappeared My Mail app was causing the screen to freeze and making the dock disappear. Now the Mail icon has disappeared. How can I fix this?"

-------

If the above does not work...

Reinstall the macOS:

Nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be. Go Here: How to reinstall macOS from macOS Recovery - Apple Support

Important:

Create a Time Machine Backup First:

Prior to the reinstallation of your macOS, be certain to backup your Mac using a Time Machine Backup. That way, you can have something to restore your Mac from, should anything go wrong with the reinstall. Use an external hard drive that is at least 1.5 the total storage capacity.

Hoping to learn Said:

"Mail app disappeared My Mail app was causing the screen to freeze and making the dock disappear. Now the Mail icon has disappeared. How can I fix this?"

-------

Use Spotlight Spotlight:

1. Open Spotlight Search: [Hold Command key + Press Spacebar key]
2. Type: Mail
3. As you type the name, it will popup

See if Mail pops up.