Dual pane replacement and/or insulated glass units is really a specialty that you should leave up to the professionals.
Not only is the exact size required, but usually the exact same thickness, overall, must be used for just the perfect fit!
Unfortunately, window manufacturers use a variety of thicknesses and components to make up the thickness and can vary from one manufacturer to another.
For instance, two insulated glass units may both be 1/2″ in total thickness but one may use single-strength for both panes while the other may use double-strength for both panes. The technical differences between the two types are minor in both appearance and costs.
Failed means the window has “fogged up” on the inside and the seal around the edges is broken which allows moisture inside the unit, and builds up condensation. The entire glass, insulated unit, then needs to be replaced.