One of my floppy disks' shutter spring got lost. Now the shutter can't slide normally. Can I still use the floppy disk?
The shutter spring in a 3.5” floppy forces the shutter back in its “closed” position when it’s opened. Losing it won’t have much impact on its day-to-day use, as long as there was no other damage to the shutter: when you insert the disk into a drive, the shutter is pulled open (which will be easier, without the spring), and when you eject the disk, the shutter will stay open.
Thus losing the shutter spring mostly means that you’ll have to close the shutter yourself when you eject the disk. You should however take care when inserting the disk, in case the shutter is slightly out-of-shape and thus harder to pull open — but that’s true of any disk really.
Alternatively, you can remove the shutter entirely, and take care of the ejected floppy as if it were a 5" one - ie. keep it in an envelope or cover.
This is perhaps not as safe for the diskette, but avoids issues with the drive mechanism.
In any case, I would plan to move the data and trash the diskette soon.
You really, really need to account for where that spring went.
If it stayed inside the diskette, it could scratch up the media. Even worse, if it came loose inside the floppy drive, it could cause all sorts of mischief. Just a few days ago I had one get loose inside the drive, and it shorted out the signal cable, causing the drive to misbehave. Fortunately, there seems to be no permanent damage, but there sure could've been!
Other than that, the drive won't care; it'll just shove the door open and it won't self-close.
That plastic front assembly can be easily replaced. It's just hooked into the metal chassis on the side - while the actual problem with the functionality is based upon the purpose of that lid: dust particles will enter the drive and from there can reach the magnetic surface of an inserted disk, when the disk's slider is opened, which will then accumulate at the R/W head and kill off the drive and/or a few floppies over time... while a "new" 3.5" floppy drive only costs around 10-15 USD/EUR/GBP.
If you mean the slider mechanism on the disk itself - it's about the same problem with the dust particles accumulating at the drive's R/W head (keeping such disks in a protective sleeve or creating a copy is suggested).