Email on my iMac is not opening up

If the Mail application is not launching at All

Close it completely - Mail

Maneuver to the Applications Folder

Hold the SHIFT key and launch the Mail Application

This is what we call Safe Launch of the Mail Application

Does it launch ?

If not >>

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Safe Mode will also eliminate Third Party Software, extensions and drivers from loading. It will only load the Minimum amount of Core Apple Processes to allow the the computer to function at a reduced Level of Performance 

3 - Does the issue present in this mode ?

4 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.

We can't help you unless you tell us what is and is not happening. Was something changed prior to this occurring?