There are several different Proform 585 designs out there, but I am assuming this is one that has 2 assist shocks to help handle the load when folding up the walk platform and one of these is now stuck.
Since those shocks normally help support the considerable weight of the walk platform they will often extend all the way out once they are disconnected and are very difficult to move back manually. The simplest way to resolve the issue is to use the components of the treadmill itself to help.
First, fold up the walk platform per the instructions in your manual (as if you were storing it). You may want to have someone help out and be sure to take care as the platform will be heavy and difficult to manage with one shock disconnected and essentially hanging off the side. Once the assembly is folded together there is still room to move the incline slightly and if you slowly raise the incline setting you will find a position that puts the extended shock and the mount in-line so you can put the shock back on. Take care that the weight of the assembly or an incline change do not pinch or trap your hand while installing the shock.
Once the shock is in place be sure to reduce the incline setting down again. Then you can fold out the treadmill again and the weight of the platform assembly will compress the shock back to the position it likely was in when you started the repair.