Site Hacked – can I back up with MySQL and start again?

You need to do a complete job of cleaning your site. Your backup may or may not be clean. Work your way through these resources and follow all instructions to completely clean your site or you may be hacked again. See FAQ: My site was hacked ? WordPress Codex and How to completely clean your hacked wordpress installation and How to find a backdoor in a hacked WordPress and Hardening WordPress ? WordPress Codex.

Change all passwords. Scan your own PC. Use https://sitecheck.sucuri.net/ before and after.

Tell your web host you got hacked; and consider changing to a more secure host: Recommended WordPress Web Hosting