The OS is damaged and won't let me go past a screen with '! ' , help!

If you search this, No bootable kernal found on disk, you will find confirmation that it's a disk problem. I remain convinced that reseating the SSD will resolve the problem, but if it doesn't, there are plenty of guides available for a DIY approach. If you go that route, you probably won't find any 16 Gb SSDs, 32 Gb seems to be the smallest after market drive that is easily available. 32 Gb is very good, 64 Gb is nice if you're planning to install Linux; 128 Gb may be overkill.

Not all SSD replacements are compatible, but this one should be: MyDigitalSSD SC2 Super Cache 2 42mm SATA III 6G M.2 NGFF M2 SSD Solid State Drive. $35 - $65, depending on capacity. For $35, I would be very tempted to order one after going to all the effort of opening up the machine.