Yeah, the same bug happened to me at least in the web version (I don't have a Windows computer, so I don't know if it's in the download version too), but after a bit of testing in a different browser, I found out that as long as you buy just about anything that is not incriseing your player damge ( crit damge might be fine but I am not sure.) from the shop, the game will run just fine. But, as soon as you buy even just one player damage upgrade, the attack on all weapons gets slow and doesn't do anything until 14-30 seconds or more have passed and If you get touched by an enemy unless you are a speed god at pressing A and D on your keyboard, then your run is over, and there is nothing you can do about it as a player. (at least as far as I know.) So until the bug gets fixed, don't buy the player upgrade in the shop if you don't want to make the game really hard for yourself.
TLDR: Don't buy player damage from the shop in the web version until the next update.