Now you have a misplaced } inside your Solve. That problem was not there yesterday. Move that } so all your equations are inside the { and } and try it again.

Perhaps you can click on Window in the title bar and then click on Magnification and choose a magnification that is greater than the default. That can sometimes make it easier to see almost completely invisible errors when they have been "desktop published" in your notebook.

In your

\[Tau]z\[Theta]2[\[Xi]_, z_] :=

definition you have a space missing between the \[Xi] and the D on the right hand side.

In your

\[Tau]zr2[\[Xi]_, z_] :=

definition you have (\[Eta]^2)[z] which should probably be \[Eta]2[z] on the right hand side.

With those changes Solve seems to return a result in a few seconds.