There are several possibilities... without more history or background it's hard to limit them. (Explore the tags at right)
You should attempt to connect to an external monitor (if you see nothing, or, if you see a normal desktop it can help offer possible solutions). The same with connecting an external USB keyboard - it need not be an Apple one (at the same time you connect to an external monitor) this too can point to possible causes/solutions.
The more information we have the more help we can be.
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