First I'd check with my ISP (internet service provider) to see if they know of any reason for this.
If your ISP is no help, then I'd download and install MalwareBytes, get it up to date, and do a scan with it.
If you're still having problems, I'd boot to Safe Mode and do a full system scan with your anti-virus program.
If none of the above works, I'd create another email account as a stopgap until you can get a computer expert to look at your system. Gmail, MS Hotmail, AOL, etc. all offer free email accounts.
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Good luck.