This all depends on what you bought, and the contract you have in place. If "XYZ" is a construction company and is responsible for the physical construction of the facilities, then you may have a contract that identifies specific damages, and the contractor may have (should have, and you as "ABC" should have verified) insurance that covers this type of loss. There may also be other considerations like how much you've paid the contractor, if you own the land/building, and other liquidated damages.
In another scenario, you rent the server farm/compute time just like you would from Amazon or Microsoft. You wouldn't "own" the servers and your contract would be entirely based on the ability to provide a service.
Either way the responsible party is "XYZ", not Milton Waddams the disgruntled employee. Your lawsuit would be entirely with XYZ as some type of contract dispute, possibly with property loss which either your insurance company or XYZ's insurance company would be on the hook for.
A lawyer would not go after Mr. Waddams, since he doesn't have the ability to pay back the type of award you are looking for, even if they are criminally responsible. You would sue XYZ and allege that they didn't take proper precautions to stop Mr. Waddams and were in violation of whatever contract you have. XYZ could then sue Mr. Waddams in a civil suit, but even if they win they would be unlikely to recover anywhere near the loss.
So a lawyer will recognize that the only reasonable recovery would be against XYZ, and wouldn't bother wasting the resources to go after Mr. Waddams.
But, to the actual question, could you sue an employee of XYZ, sure, you can, but the case may be thrown out. Since the employee is a representative of XYZ, and is technically working on behalf of XYZ, the company is ultimately responsible. If Mr. Waddams had gone into your office and stole your petty cash box, you could probably sue them and win. If they entered your facility and stole all your IP, you could also sue them, but whatever you win will be difficult to collect.