I downloaded a software from internet...I did a mistake in turning off my antivirus. After download, I noticed that all my files has a ".meds" suffix in the extension name. I can't open now the files even I rename / deleting the ".meds" suffix. Thanks.
Sounds very much like your system has been infected with a ransomware virus, but it's not one that I recognise. You could try asking on https://malwaretips.com/ to see if it's one that someone has come across before.