Thank you for posting.
Since this was working previously, and there have been other issues on this machine sending mail, it is likely a computer configuration change is affecting FileMaker.
If Lotus Notes is no longer set up as the default mail program, FileMaker Pro's Send Mail won't work. Try using SMTP to send a test message: In the File/Send >Mail.. panel, use the drop down for "send via" and select SMTP Server instead of e-mail client. Specify the SMTP server settings in the same way you would when configuring a mail client. If you are able to mail in this manner, you can focus on fixing the e-mail client installation/configuration to resolve the failure of FileMaker to send mail via that client.
If you encounter any messages while testing, they may help to narrow-down the cause of the problem as well. Let me know if you discover any additional information.