You have placed yourself in a minefield. Having a crush on your best friend is a very dangerous territory. The best advice I can give you, based on my personal experience, is to suppress the crush. Forget about it, keep them as friends and move on. There is only a 1% chance that this endeavor succeeds. It’s not worth losing a friend over.
But if the crush on your best friend is serious, I would say that you should tell them how you feel and make the next move based on their reaction. If they do not reciprocate the feeling, ask them to bear with you while you move on from them and apologize. I know it’s stupid to apologize to a best friend but it's necessary for your own uncontrollable heart. Assure your friend that you will be fine and might require their help to move on. Tell them that you won’t let this come between their friendship. I know it seems a bit dramatic but as I told you before it's easier this way. Your friendship with them will change because of it, but eventually, it’ll get alright.
If you happen to stumble upon the rare chance in which they do reciprocate the feeling, then my friend, you are in Candyland. There is a third thing you can do. Hide your crush from them. Non-reaction is also a path you can take in this situation and wait until the crush fades.
If you have a crush with your best friend then you should share with her/him & if she/he accepts your offer then this relationship will run forever because your partner already knows all-thing about yours.
It can be hard when you suddenly realize that you have a crush on your best friend. It can make you feel a bit uncomfortable especially if you know that your crush does not think of you the same way. You have two options. The first option is you are not going to say anything about it. You will just care for your best friend from a distance.
The second option is you will let your best friend know about it so that you will know what your best friend’s feelings for you are too. If your best friend tells you that he/she does not feel the same way, then you have to accept that. You both have the option to continue the friendship or not.