Most machinery will use a consistent standard for all the fasteners. If one is inch standard (in the US that's generally called an SAE bolt) they all will be. If one is metric they all will be.
Find another bolt on the same machine that looks like about the right size. Remove it, and try to screw it into the hole where the missing bolt was. If it goes in snugly (at least two or three turns) take the bolt to the hardware shop.
At the hardware shop, find a nut that the bolt screws into. Buy a matching bolt (thread size) that goes into the nut you found but with an appropriate head for the missing bolt location and with your estimated correct length. (And as a courtesy to other customers, be absolutely certain that you put the nut back in exactly the same bin it came out of at the hardware shop.)
Take both bolts home and put them back on the machine.
Before tripping to the hardware shop, it's also helpful to look carefully at the "hole" where the bolt is missing. If the entry to the hole is "chamfered" you need a screw/bolt with a "flat head." (It's the chamfer that's important, and flat, crowned, truss, etc varieties of countersunk heads may be found and are usually interchangeable.)
If there's a "circle" around the hole where the bolt is missing, deliberate or from wear, you probably need a washer. If there are little "peck marks" or another similar "scab" you may want a lock washer.
If the hole has a bottom in it, it may be important that the length of the replacement screw be "not too long," but if the thread goes all the way through it usually is necessary only that the replacement be "long enough," although "bolts" are threaded only at the end, so one too long may run unthreaded bolt into the threaded hole and damage the thread in the hole. Being "close" to the right length is of some importance, but "exact" isn't usually critical.
Machine "Screws" are threaded all the way but are generally lower strength in the kinds found at the local hardware. They may be called "stove bolts" in the US, and generally shouldn't be used for anything that shakes, rattles, or makes noises in use without good reason. The distinction between "screws" vs "bolts" is rather vague though (even in the official "standards"). and it's not too important what the seller calls them.