There can be nothing more confounding than having to put a screw or bolt in a hard to reach hole, except when you try to do so, and the screw drops deep into the gubbins of the machine. You can see it (if you are lucky), bvut you dont have the reach, or the gaps is far to tight for you to grab it. what can you do?
Many screwdrivers have magnetic tips - if you are not working in the control board areas of robots (where the sensitive electronics are), then these may help in retreiving the screw. However - if the place where they have landed is ferrous (responds to magnetism) as well - this wont help. If the screwdriver is a bit short, there are actually very handy pen-size telescopic magnets (you can also get telescopic dentist style mirrors as well).
If the screw/bolt is now magnetic, or it has magnetically responsive material behind, you wont be able to use a magnetic means to get it. Some PC tool kits come with an extraction tool designed for this - it is generally a long pen sized tube, with a button on one end, that operates a crude grab claw on the other end. They are great for stuff thats not too deep - but may not always work.
An fairly easy trick is to get a pen, or a longer stiff rod of some sort, and apply a nice mass of blue-tac to the end. You can then push this into the screw, and retreive it. This also works for pulling out the head-pulling bolts used a lot in self-assembly furniture when they are seated to deep in the hole.
You may be able to use the extraction tool or blue-tac method to actually insert, and get the first few turns in on a screw in an awkward place. This is a technique worth considering before it drops and winds you up.
If you cannot see it at all - this can be a real problem. It is generally not a good idea to leave a screw rattling around, especially when it may connect with the electric control systems and short something, or get stuck in a gear system and jam it. When you get this, you need to lift the item, and tilt it to try and roll the screw to somewhere more acessible - depending on the item, and where the screw may have become lodged - this is not easy.
Sadly - there sometimes comes a time where the only way to get to the screw is through disassembly. This is a good point to mention that it is worth in your design making most panels removable for maintenance, and for just this kind of occasion.
Whatever you do, dont let it frustrate you. Your best approach to any setback, is to slow down, take a step back and think it through.
Time to build your own robot! Using a Raspberry Pi with parts and some time, you can use my book to learn how to make and program a robot with automatic behaviours.