Hello,
I'm going to assume by that message that you mean the taskbar. In that case, OneDrive doesn't support being opened as an actual app from the taskbar, it's only meant to be used as an app on the side (a pop up on the side).
However, if you click the up arrow (or side arrow depending on taskbar orientation), you should see the OneDrive icon in the overflow menu. Click and drag the icon back into the taskbar (you should see a line near it).
Does that fix your issue?
I hope to hear back from you.