Cant open mail

Please can anyone help me (preferable in words of two syllables or less) as I am computer illiterate.

I had a message saying that I did a force quit, which i probable did as sometimes I get frozen. Now

when I try to open mail I get a box saying 'there is a problem with your mailbox, click and rebuild index next time i open mail it will reindex.'

I have tried this numerous times without success. Have been unable access emails for over a week now.

Have even thought of buying a iPhone or iPad to see if i could access them that way, but would be a very expensive experiment and probably wouldn't work anyway.

Any suggestions gratefully received

am using iMac high sierra