This is quite normal, I think. I take a break, do something refreshing, like a short nap, meditation, or sometimes a walk outside helps. Then I try again.

If the motivation is still tanked, I weigh up how urgently the task needs to be done, and if it can be broken down into steps. If it must be done and I can do some of it now, I'll find a tolerably small piece of the work to do, and do that. Then after that's done, I'll reassess. Often doing a little leads me to finishing the task, but sometimes I'm still tanked and I just let it go until the next day, but I'm reassured that I've at least tested the waters.

Motivation ebbs and flows. Have a listen to Huberman lab on YouTube, he has some interesting talks on the science of motivation and some tools to help.