What can I do if I have a lifetime warranty on a bow window but can’t get a call to the contractor for replacement of a glass panel?

If the contractor doesn't respond to your calls, the first thing to do is to check whether they are still in business: you can try an internet search, or even driving by if they are close. If they are out of business, your warranty is probably worthless. If they are still in business but won't respond to you, try sending them a letter, sent some way you can track it and prove delivery, describing the problem; enclosing a copy of the warranty and quoting from it, to show that you are covered; showing proof that you'd paid for the warranty (e.g. a canceled check); asking that they honor the warranty; giving them a reasonable, but short time to contact you (like 10 business days) and enclosing your contact information; and warning them (respectfully and professionally) that if they will not honor the warranty, you may be forced to take any or all other appropriate action, including contacting the better business bureau, contacting your state's consumer protection agency, and/or suing. If they still will not respond in a satisfactory way, then your next step is to escalate to one or more of those options--if you choose to sue, one option is to sue in small claims court, acting as your own attorney (pro se) to keep costs down.