Hoster disabled website saying high cpu load (39%) from index.php. But could not specify the exact cause. He enabled it shortly and i saw the "whos online counter" raising steadily until 2500 (!) something is obviously wrong with the site and must be checked and optimized by a professional.
1 step: I give you the ftp login, you check on zen cart files, mysql, everything you can to find the problem with this, if zen cart must be optimized do it, if its malicous code - find it
2 step: If you found the problem or not, you transfer the site to a new vps hosting make it online and debug while running if needed
3 step: Upgrade zen cart to latest, patch, optimize databases, harden security.
You must be zen cart expert and start working asap !