Robb could have sent the ravens, but the question is who would have gotten them?
Winterfell had fallen, so there’s no Stark to coordinate the reconquest; the Northern lords are thoroughly distracted by the Hornwood Crisis and even when they get past that, the failed attempt to retake Winterfell further scatters the loyalist forces.
These are all dominos that have to fall for the North to fall, because as we see in ADWD, even with everything going against them, the Northmen still put Moat Cailin under siege, with the crannogmen attacking from the south and the Ryswells and Dustins from the North, and then Ramsay and Roose coordinating from north and south of the Moat. If Winterfell had not fallen or had been recaptured, Moat Cailin would no doubt have fallen faster.
The only armies the north has left in the north is garrisons left behind to protect their castles. Theon was able to capture Winterfell by attacking the coast and drawing the garrison away from Winterfell. After that happened, I cant imagine Manderly wanting to send his garrison to take back Winterfell, thus leaving White Harbor undefended. Especially because 1) White Harbor is, well, a harbor and therefore much more susceptible to an Ironborn attack and 2) the only reason Theon was able to take winterfell in the first place was because no one was expecting it, without a robust offense or the element of surprise, retaking winterfell would be next to impossible.
^ This isn’t quite accurate.
When Theon captures Winterfell, Rodrik Cassel is able to throw together an army of 2,000 men in short order, including 600 men from Winterfell, but also 300 Cerwyn men, and 1,100 men from the Tallharts, Hornwoods, Flints, and Manderlys, who provide “a dozen barges upriver packed with knights, warhorses, and siege engines.”
Add to those 2,000 another 3,500 from the hill clans – Wulls, Norreys, Liddles, Harclays, Flints, and Burleys – who show up to help Stannis take Deepwood Motte, plus another 450 Karstark men, 400 Umbers, and an unknown number of Mormonts and Glovers who join Stannis’ army after Deepwood Motte.
That brings us up to 6,350 men. Add to that the forces of House Manderly that Wyman held back at White Harbor (he brings only 300 men to Winterfell) – the fighting men of the warships he’s hidden up the White Knife, the “more heavy horse than any lord north of the Neck,” the dozen petty lords and hundred landed knights.
So it’s not that the North is out of fighting men, it’s that its fighting men are disorganized, split between many different factions, and lacking central leadership to bring them together into one force and point them in the same direction.