All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
You should not have used this money in other than the purpose which you collected it for. This is prohibited transgression and breach of trust, and you must hasten to repent to Allah, the Exalted, from it. If the purpose for which they donated still exists and that person is still alive, then give this money to them; otherwise, give the money back to the donors. The difficulty of facing them back is not an excuse to prevent returning it. You are not obliged to tell them what you did; rather, what matters is to return the money to them in a way by which they realize that they own it. As for the donors whom you do not know, it is sufficient to spend what they donated on a similar case, such as a cancer patient in need of financial aid.
Allah knows best.