is it not answered in the resident eil wiki already i sugggest you check there ! ...Ada was struck by a rapidly-mutating William Birkin and was critically injured. During the ensuing confusion, Ada was seriously injured protecting Leon and presumed dead (see:Canon in Resident Evil).
She managed to get out of the laboratory and made her way to the Apple Inn where she was due to meet with an Umbrella worker. When she got to the room, she found that the man had already committed suicide. Even worse, Wesker was watching her live on a computer. When Ada shows him the G-virus sample she recovered from William Birkin, he told her there were moments left until the city was wiped out by a missile. He helped her by telling her a helicopter sent to evacuate the last of Umbrella's workers, who were revealed to be Sergei Vladimir and Ozwell E. Spencer was set to leave the city shortly.
Providing Ada with a Hookshot, Wesker expressed his interest that she survived with the G-virus sample. She fought her way past the hoard of Zombies and defeated a large Tyrant monster on one of the highway bridges leading outside the city. She then used her Hookshot to grapple onto the Umbrella helicopter as it passed overhead. While she ran as the copter flew away, a Hunter chased her and jumped to grab her as she was lifted into the air. The Hunter failed to grab Ada by mere inches, and instead got her slipper.