This is possible on the PC version by making use of the console which you can open by pressing ~. You can then jump to any level with the map x console command, where x is the code for the level you wish to jump to. The list of levels (most levels are split to several sections) and console commands to access can be found on GameFAQs:
Black Mesa Inbound c0a0, c0a0a, c0a0b, c0a0c, c0a0d, c0a0e
Anomalous Materials c1a0, c1a0a, c1a0b, c1a0c, c1a0d, c1a0e
Unforeseen Consequences c1a1, c1a1a, c1a1b, c1a1c, c1a1d
Office Complex c1a2, c1a2a, c1a2b, c1a2c, c1a2d
"We've Got Hostiles" c1a3, c1a3a, c1a3b, c1a3c, c1a3d
Blast Pit c1a4, c1a4b, c1a4d, c1a4f, c1a4g, c1a4i, c1a4j, c1a4k
Power Up c2a1, c2a1a, c2a1b
On a Rail c2a2, c2a2a, c2a2b1, c2a2b2, c2a2c, c2a2d, c2a2e, c2a2f, c2a2g, c2a2h
Apprehension c2a3, c2a3a, c2a3b, c2a3c, c2a3d, c2a3e
Residue Processing c2a4, c2a4a, c2a4b, c2a4c
Questionable Ethics c2a4d, c2a4e, c2a4f, c2a4g
Surface Tension c2a5, c2a5a, c2a5b, c2a5c, c2a5d, c2a5e, c2a5f, c2a5g, c2a5w, c2a5x
"Forget About Freeman!" c3a1, c3a1a, c3a1b
Lambda Core c3a2e, c3a2, c3a2a, c3a2b, c3a2c, c3a2d, c3a2f
Interloper c4a1, c4a1a, c4a1b, c4a1c, c4a1d, c4a1e, c4a1f
Gonarch's Lair c4a2, c4a2a, c4a2b
Nihilanth c4a3
Conclusion c5a1
You can give yourself weapons and ammo with the following console commands:
give ammo_ARgrenades
give ammo_9mmclip
give ammo_357
give ammo_9mmbox
give ammo_crossbow
give ammo_Egonclip
give ammo_Gaussclip
give ammo_9mmAR
give ammo_RPGclip
give ammo_mp5grenades
give ammo_buckshot
give weapon_python
give weapon_handgrenade
give weapon_tripmine
give weapon_snark
give weapon_357
give weapon_crossbow
give weapon_crowbar
give weapon_9mmhandgun
give weapon_gauss
give weapon_hornetgun
give weapon_mp5
give weapon_rpg
give weapon_shotgun
give item_healthkit (health)
give item_suit (HEV suit)
impulse 101 Instant (all weapons, full ammo and armor)
Most commands are self-explanatory, comments are inside parentheses (()).
Best Answer
No; NPCs generally do not respawn in Half Life. I'm not aware of exceptions, so cleared areas should remain clear for your backtracking pleasure.