@Vasconia
Firstly you need to eliminate any chance that the USB cable or PC USB port you've been using are the problem. Try with several different miniUSB cables, use only REAR desktop PC ports without a hub and even try a different PC or laptop. Ensure the computer has Garmin USB Drivers loaded as well, that's absolutely essential: [Only registered and activated users can see links. ]. Your device should still have preboot mode available and be visible in Device Manager as "Garmin Device" even though it may not appear in explorer in Mass Storage mode:

[Only registered and activated users can see links. ]

Try to put it into preboot by holding the top LH corner of the screen with it completely OFF (you may have to force it off by holding the power button down for up to 30 seconds) and then connecting it to PC, listen for the windows ding-dong connection sound and have Device Manager open to see if it appears there as shown above. This test is to confirm whether a software solution is available for the device. If you still cannot see it in Device Manager after doing all my suggestions then read on.

My suspicion is that the device has a damaged USB connection and if it indeed has then it's either a component on the PCB that has failed (very unlikely), or the socket itself has a bent pin or other damage (somewhat unlikely), or one or more of the socket's solder connections to the PCB have broken (far more likely). The last is very common on 14xx devices. The first is not economically viable to fix for an old device, the second and third might be easily fixed depending on your technical ability. A hardware fault is almost certainly what has occurred when neither Mass Storage mode nor preboot mode are available.

An explanation about the purpose of Cure firmware. It cannot boot a device and indeed when it's loaded the device is meant to stick on the logo screen. That's because the purpose of loading cure firmware is to re-enable Mass Storage Mode on a device which has lost it because of a faulty file which cannot be loaded on boot. When the device can't load the essential, but corrupt, file during the boot cycle it typically doesn't get far enough into the boot cycle for MSM to be enabled however pre-boot mode is typically still available so cure fw can be loaded, that then bypasses any faulty file being even called on to load but allows MSM to be regained so a corrupt file can be removed from the file system or a reformat of it done. These's good reason why we don't load cure fw as a GCD but only as an RGN in preboot. If a device doesn't have either of MSM or preboot modes available it's almost certainly got a hardware problem. You can't fix a hardware fault with a software solution anymore than you can fix a blown-out car tire by pumping air into it.

If my suspicion of a faulty USB socket is correct, all you've done is introduce another problem. So let's remove that first but would you please confirm beforehand that you don't have pre-boot mode available?