iMac shutting down in start-up loading.

I have an iMac running macOS 10.13.6 (a much older model).

Every time I try open it it just closes after 80% loaded (estimated load time). I've tried to restart it while it was loading but it just took even longer to shut down. It has been doing this for about 2 years and I haven't been able to get it to a place where they may be able to fix it. It may be something to do with the SSD as before it broke I loaded up quite a heavy file. I've come here for advice because I don't want to spend a lot of money for something I could just fix with a key bind or something else.