|Have you done a power reset on both the modem and switch in sequence?|
Power down all devices. Power up the modem/router then after it has settled power up the switch. This will allow any auto-adjusting ports to accommodate themselves to whatever type of ethernet cable you have between the modem/router and the switch.
Rule of thumb is Like devices require crossover cable. Unlike devices use Patch.
Modem & Switch are "Like" devices. Switch & PC are "Unlike"
Not all modems, routers and switches have self-adjusting auto-sensing ports. If you do not have devices with self-adjusting ports you must use the appropriate type of cable. If the ports are self-adjusting they can only do so following a power reset.
Goin' Fishin' (Some day)