If the frame is warped, the glass is cracked, or the window has failed multiple times, replace the whole unit or the entire window. Also, very old windows (20+ years with original seals) may not be worth repairing—newer low-E, gas-filled units will outperform a reseal significantly.
That subtle draft by your favorite chair. The mysterious condensation fogging up your glass. The gradual hike in your heating bill. getting windows resealed
Never caulk a window shut. If the window is meant to open, the moving parts need weatherstripping, not caulk. If the frame is warped, the glass is