Your contractor is due back in a week. And if you can contact him, maybe sooner.
At that point, your company will be back to 2/3 staff.
Meanwhile, don't panic, don't do anything rash. Your 29 year old partner is down with a heart attack - you must be freaked. Dealing with mortality and surprise and fear along with the sudden increase in workload. This is not a time for drastic decisions.
So, you contact all your clients and let them know what's up. This is a big, fat, hairy deal, and most of them will understand and work with you.
Reprioritize the work in progress, don't accept any new work. Find out what's critical to each client and what can get done next week or the week after. Outsource anything you can. Get creative - call in favors - draft your friends, SO, kids, pets... Even if they can't really help, you can probably use the emotional support, and (trust me on this) your support network feels better when it gets a little use. If you don't have a support network, get one as soon as this crisis is over, or you will die. Medical fact.
Next week, when the contractor is back in house and you have the drop-dead critical stuff up to a survivable state, get enough rest to clear your head, and decide what to do next. You will have more information at that point.
Electrical banana is bound to be the very next phase