It is already the second time LIRR failed since we moved to long island.
Here are the back up plans going home:
1) Subway alternative… Take #7 train to flushing. Then take Q12 (final stop Little Neck) N20/N21 (some ends at Great Neck, but some goes to Manhassett, Roslyn, Glen Cove, Greenvale, Old Westbury Hicksville).
2) If LIRR partially functioning… Port Washington line and Oyster Bay line can be backup of eachother, N20/N21 connections Great Neck of Port Washington line with Roslyn, Old Westbury etc of Oyster Bay line.
If LIRR Montauk line is functioning, take Montauk line to Lynbrook, then N25 to Great Neck, then other buses from Great Neck, if needed.
3) Express bus… Take QM3 from middle town to Little Neck.
34 St & 3 Av
34 St & Park Av
34 St & Madison Av
35 St & 6 Av
6 Av & 41 St
6 Av & 44 St
6 Av & 48 St
6 Av & 55 St
57 St & Madison Av
57 St & 3 Av
There are only three afternoon shuttles. click for schedule :
4) China town van… Go to Chinatown, enjoy a dinner and take a van Chinatown-Flushing. ticket is $3. Then take Q12 or N20/21.

#7 train is the best backup. Other alternatives are more vulnerable to traffic chaos after the storm.
If LIRR is not functioning, show your LIRR monthly pass and you should be get free ride on #7 train, M12, N20/21 and QM3 bus. (tried everything except QM3)

5) Just stay… Find a place to have a drink, stay with a friend in Manhattan, enjoy your night.