I have just gone to open one of my clients, but it is showing as deleted, but I haven't deleted it? How can I get it back??

Hello Tracy-accounts-b, welcome to the Community page, you probably just marked the client as inactive, you can check it from the clients tab>tick the small gear icon>tick include inactive box and it will show all clients, if your seeing the client as Inactive, then you can click Make active. You should check if the client is still on your WSB too from the gear icon>Subscription and Billing it will show there the billed clients if they were on your WSB to begin with.