The recipe only called for 1tsp of yeast, but when I opened the package I poured almost the whole thing (2-1/2 tsp) into the water. Now it is just sitting there not rising. How can I fix?
"Sitting there not rising" is a very unusual description if you used 2.5 times the required amount of healthy yeast. The usual symptoms would be that your dough starts rising (slowly at the beginning, just as normal), then rises a lot, developing a strong smell with a few unpleasant notes (ammonia, but also a bit sulfury) and then ends up very weak, deflating on touch, impossible to be shaped into loaves (or if you do so, it doesn't rise again).
If you just kneaded the dough and are watching it waiting to rise, it might not yet have started rising. In that case, there is one way to save it: make a second batch without the yeast, and knead the two together. Do it right away, before the problems have started. You can freeze half the dough if you don't need so much bread now. Continue making the bread as usual. Most recipes will have enough tolerance that you don't have to account for there being 2.5 times the yeast and only 2 times the dough, but you can also make 1.5 additional batches if you can handle it equipment-wise.
In all other cases - for example if you already shaped everything as usual, hoping that the deflated dough will rise again during the second proofing, or if the dough has been sitting for a long time not rising at all (which will indicate some problem with the yeast beyond the amount), there is no way to save this batch. You have to start over, following the instructions exactly. And if you suspect the yeast to be a problem (e.g. it spent two years open), it is probably best to get new yeast too.
If you're at a step where you mix the yeast into warm water (possibly with some sugar in it) and wait to see if it bubbles, that's called proofing the yeast. You're basically rehydrating the yeast, to get it growing again. The bubbling confirms the yeast is still alive.
If it doesn't bubble, then the yeast isn't growing. Putting 2.5x the right amount of yeast in the water probably isn't why. Possibilities:
To fix your yeast ratio in your proofing solution, the easiest thing to do is add 2.5x as much of the other ingredients, then only use 40% (1 ÷ 2.5 = 0.4) of the mixture. Discard the rest, or immediately use all but the last 20% in a second batch. The yeast concentration is probably not very sensitive, so you could double the other ingredients and then use half of it.