Do you lose money if your bank is robbed?

No. Nearly all banks have private insurance that covers them if they are robbed. (It is not the same as federal deposit insurance.) In addition, most banks take elaborate measures to safeguard the cash and other valuable items left in their care.
Bank vaults have long been protected by reinforced concrete walls, time locks, and metal alloy doors that resist drilling and explosions.
At one time, armed security guards stood watch over banks, but today most banks seem to have decided (wisely) that they would rather not expose their customers and employees to gunplay.
Shotguns and revolvers have been replaced by closed-circuit television cameras that maintain a constant watch over everyone who enters or exits the bank.
Another innovation is the exploding dye pack. In certain cases, bank employees are able to place a package of red dye in with the robber’s stash of stolen cash. Later, when the crook opens the stash, the concealed dye pack explodes, covering the robber and the ill-gotten money with dye that won’t wash off.