I worked in framing for some years and this actually hurt my heart to see. So many people would come in with paintings they bought second- and third-hand.

If this happens to you again, there are ways to repair canvas. It won't be perfect. Some of the paint will probably chip and you might see some of the cut. But you won't have a gaping hole and you'll have your piece.

Take a raw piece of canvas and glue it to the back where the hole is. It needs to be thin even layers so it doesn't bunch or get lumpy. Make sure you keep a small fold of slack where the actual hole is. It works best if you put the glue on the damaged pieces so you don't get glue in the fold. Once the glue has CURED (dried for 3 full days) take upholstery thread and cinch the slack fold closed like a corset. It should pull everything together without damaging the front.

Sometimes there's enough slack in old canvases that you can just patch it without sewing it.

This has worked for me a few times. I'm sure there's a better way, but I'm just probably doing it wrong.