All stealth aircraft have small radar signatures, not invisible, and an enemy with an active air defence network and enough time and knowledge can use ambush tactics to destroy them. This is exactly how the F-117 was downed by Yugoslavian Serbs:

They observed the area where the bomber was regularly approaching from for bombing missions. NATO complacency assumed using different aerial routes wasn't necessary.
Yugoslav missile systems were modified for quick redeployment, allowing them to operate despite heavy NATO air cover.
They explicitly calibrated early warning radars to detect the incoming stealth aircraft, reducing its range from 200km to just 24km. This gave an express warning of when to activate more precise tracking radars.
The ambush tactics, alongside the express warning given by the early warning radar, meant they were tracking the F-117 in question at the moment it was opening its bomb bay doors, allowing for a precise targeting lock by the S-125 radars.
Luck would have it that electronic warfare planes were grounded due to poor weather. This was known by Yugoslav commanders who had penetrated NATO communications and had sympathetic civilians revealing info on sorties. This helped the S-125 targeting radars to work more effectively and for longer (knowing they were unlikely to be detected) which ensured the kill.

An enemy that is complacent, gives up operational intelligence, and otherwise ignores basic operational procedures will never be totally invisible to the right combination of luck, planning, and cunning.