Moving this discussion to the Photoshop forum.
Making a quick stab here but is there any chance you happen to be on a layer where the lasso or eraser tool cannot do anything? (E.g., an adjustment layer). Can't tell you the number of times that has bit me.
One clue for this is that rather than see the normal cursor, you get a "null" symbol (the circle with a line through it).
Please let us know if this is the problem. Otherwise, can you provide more detail as to what you see when you are trying to use these tools. And, while this may not have anything that can help with this issue, providing your OS and verifying which version of PS you are using might be of assistance.
Thanks so much for your reply. That is a good guess but it's not the case - I am always on regular layers, and other similar tools continue to work (like the magic wand or the box selection tool). I have also made sure to deselect everything just in case I unknowingly selected something and can't use the tool because I'm outside the selection.
When the tools stop working, I still see the icons as my cursor the way I would if they were working, except nothing happens when I try to use them (for the lasso, the 'marching ants' selection line does not show up, or nothing erases when I use the eraser). I am using Photoshop CC on a 2017 iMac running macOS Sierra. Any other ideas?
I've also had some "stupid photoshop tricks" events, not this but there's times when a brush just doesn't seem to want to work.
Have you tried quitting and restarting PS? Restarting your computer? Have you tried resetting your PS's preferences? I'd suggest trying those in that order (from least to most time intensive) and seeing if any of these have any effect.
Please let us now if this does it for you or not.