You get a bit of a grace period to immediately launch an assault on the nearest enemy base. However even when the bar fills up, a 20-day countdown timer begins. The only way you can really lose XCOM 2 is if the Avatar Project is completed. Many soldiers died bringing you these invaluable tips, so that others may live.
You and the brave men and women fighting for XCOM are our secret weapon. The aliens continue their stranglehold over our planet.