Is one missing roof tile serious?
A single missing tile can be important because it leaves the layers underneath more exposed to wind and rain. It may remain a small repair if dealt with promptly, but leaving it can allow water ingress or neighbouring tiles to move, especially during further bad weather.
Why one tile matters
A pitched roof works as a system. Each tile or slate overlaps its neighbours and sheds water onto the course below, and the underlay beneath is a secondary defence rather than a waterproof layer in its own right. Remove one unit and both the weather protection and the wind resistance of the surrounding area are reduced.
That is why a gap can widen. Wind gets under the exposed edges of neighbouring tiles, and rain reaches underlay that was never intended to be the outer surface for long periods.
What a repair usually involves
Replacing a single tile or slate is normally a straightforward job, provided a matching unit can be sourced and the fixings and battens underneath are sound. A roofer will check the surrounding area at the same time, because a tile rarely moves without a reason.
If the same area keeps losing units, the cause is often the fixings, the battens or previous work rather than the tile itself, and that is worth establishing before repeating the repair.