Did you make the center or the arc coincident with something in the sketch or part? If so, you need to delete that coincident relationship before you can move it. SolidWorks does have this unfortunate problem with some relationships, though. If you made your center point coincident with the end of a sketch line, Display/Delete Relations won't show you any relations, so you can't delete it. You have to delete the line, trim the end off, or split it and then delete the end.
I'll admit that I'm experimenting. Attached is a drawing of a quarter-circle shelf. On the shelf are lines indicating two vertical riser boards, meeting at a right angle. I defined the center lines, then the inner and outer edges. The inner edges meet. So far, so good. I also tried this in a 2-D sketch, using offset entities to define the board edges. The first problem is that I couldn't get offset-entities to work in the 3-D sketch.
The ultimate intent is to model the thing in 3-D. I'd like to extrude the shelf a half-inch. So far so good. Then I'd like to work with the risers, modeling them separately and adding appropriate cuts to the main board.
Note the fiddle bits where the two risers meet at the edge of the shelf; part of the edge of the board (and I have the same problem at the other end of each riser) is defined by the curve of the shelf. I can't extrude the risers until I close the circuit.
* I can't cut the shelf curve, because I can't figure out how to make the cut-entities tool select the curve at the junction of the curve and the line. It keeps wanting to select the line.
* The problem previously described occurs when I try to draw another curve over the top of the first, joining the two outer edges and using the same center as the original inner curve. If I zoom out far enough to see the center point (0,0,0) I can't really see the two end points. The arc tool doesn't want to do it right anyways, it keeps putting the center point at some remove on the z axis. Now with any reasonable tool I should be able to click on the arc, pull up the property manager, and manually set the various parameters. It won't let me do that.
I also included the original 2-D sketch. Is there any way to turn this into a 3-D sketch so that I can work on the Z-axis (with something besides the boss-extrude tools)?
Thanks for your patience.
Until you get more comfortable using the sketching tools, I would stay away from 3D sketching. It is infinately more complicated trying to keep things constrained in a 3D sketch.
I would suggest breaking up the individual components into their own sketches, and make the 3D geometry as you go. Start with your base shelf (removing the riser lines) and extrude that. Then using the top face of that extrude start a new sketch for one of the risers. At the corner where they meet, you can use "Convert Entities" to copy the curve of the shelf into the new sketch and then trim away the excess. You can then extrude the first riser (making sure to uncheck the Merge options), then do a mirror to create the second riser.