If you have 0 motivation, it could mean you're just getting bored or saturated or stressed out or simply overwhelmed. In any case, do NOTHING for a day or two, don't feel guilt (that's KEY), tell yourself you're taking a break, play games, go ride your bike, walk in a forest, go to a restaurant, whatever suits you. Make yourself feel good. Take a break. Then the next day start by doing something very very simple. If you're working on a very hard problem then run a simple experiment to validate a case, corner case or not. If you feel like you don't feel like it, then just do some silly 5min task, it may be enough to get you restarted and just feel better. Feel good, enjoy life, then get back to work. Changing Job may do the trick too. Or take some vacation, the idea is to just break the cycle.
Some days I feel "normal" but some days I have 0 motivation at all. Not sure if that's a normal pattern or not but its frustrating to deal with.
Might be worth talking to a Dr about but not sure what they would even do or say.