How can anyone help you when we don't know what model keyboard you have. There can be different designs on different models.
But, in general use a little logic and look at the underside of the space bar and look at where/how it mounts. Usually it will only go on one way, correctly. (It should not have been disassembled for cleaning).
I am not a Dell Employee
Dell forum member since 2002
Home Built PC with ASRock Z77 Professional, i7 3770 CPU, Windows 10 64 bit Pro. SSD drive. Sonar Platinum Recordng Software.
Member of Nashville based R.O.P.E.
It's fixed! All I had to do was remove the skinny plastic bar from the back of the spacebar, insert that little part into the keyboard first, and then snap the spacebar on top of it.
Incidentally, the keyboard is a Dell RT7D50 made in China.
Oh goodie, another one
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