Messages in my phone is not opening at all

The app cannot be deleted so...

First try a Forced restart

Force restart iPhone – Apple Support (UK)

If no better

Backup, restore to factory,  and   test, then restore from backup:- 

(If the phone needs attention you will have to do this anyway)

If you can import your media to Photos or iTunes in a computer and copy important documents to a cloud server it will give you some resilience in case the restore from backup fails, or you need to ditch the backup and start as new. 

(1)          How to back up your iPhone, iPad, and iPod touch - Apple Support

(2)          Restore your iPhone, iPad, or iPod to factory settings - Apple Support

(3)          Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If the problem remains then do step (2) again but set up as new without restoring any backup.

 If it fixes it then it is probably a rogue app like VPN

Otherwise arrange a repair

iPhone Service & Repair - Apple Support

Settings, Messages, Turn off iMessage

Settings, Face Time, Turn off Face Time

Restart the phone.

Settings, Messages, Turn on iMessage, SMS, MMS

Settings, Messages, send & Receive, select your number which should now be there.

Settings, Face Time, Turn on Face Time