She's gone thin and runny

She's over-fermented. As a starter passes its peak, the acid breaks down the gluten holding her structure together and she goes slack and soupy. If she's also smelling sharp and has hooch on top, this is it. Feed her, and use her sooner next time.

Or your ratio has drifted. If you've been eyeballing feedings, you may be adding more water than flour without noticing. A standard starter is equal weights of flour and water, and it should be thick, like a stiff batter that falls off a spoon in clumps rather than pouring.

What to do. Discard down to a spoonful and feed by weight, not by eye. Equal grams of starter, flour, and water is the simplest place to reset to.

If she's still loose after a couple of feedings by weight, use a little less water, or switch part of the flour to whole wheat or bread flour, both of which absorb more.