unerreichbar
inaccessible
Not reachable spatially or physically; inaccessible.
Nach dem Erdrutsch war das Dorf für mehrere Tage völlig unerreichbar.
After the landslide, the village was completely inaccessible for several days.

unreachable
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inaccessible
Not reachable spatially or physically; inaccessible.
Nach dem Erdrutsch war das Dorf für mehrere Tage völlig unerreichbar.
After the landslide, the village was completely inaccessible for several days.

The word is made up of the prefix 'un-' and 'erreichbar' (reachable). 'Erreichbar' comes from the verb 'erreichen' (to reach). 'Unerreichbar' therefore literally means 'not reachable', which can refer to places, people, or goals.
Der Gipfel des Berges ist bei diesem Wetter unerreichbar.
The mountain peak is unreachable in this weather.
The context makes the meaning clear: with places, it is about physical accessibility. With people, it is about contact by phone or email. With goals, dreams, or records, it is about achievability.
Das entlegene Bergdorf ist im Winter oft unerreichbar. (Ort)
The remote mountain village is often inaccessible in winter. (place)
'Unerreichbar' is very often used with the preposition 'für' (for) and an accusative object to specify for whom or what something is not reachable.
Diese hohe Qualität ist für die Konkurrenz unerreichbar.
This high level of quality is unattainable for the competition.
impossible
'unmöglich' and 'unerreichbar' can both express that something is not possible. The difference is in the perspective. 'unmöglich' is generally used when an action, plan, or event is not feasible. 'unerreichbar' is used when you cannot reach a goal, a place, a person, or a level. This can be spatial, by phone, or meant figuratively. When it is about feasibility in general, 'unmöglich' is more natural. When something is too far away, not contactable, or too high as a goal, 'unerreichbar' is more precise.

Maria ist seit dem Morgen unerreichbar, weil ihr Handy ausgeschaltet ist.
Maria has been unreachable since this morning because her phone is switched off.
Ohne Passwort ist der Zugriff auf die Datei unmöglich.
Without the password, access to the file is impossible.