Labour lost in 2015 for the same reason it lost in 2010: it — like the other main parties — did not tell voters the truth — or what policies they had to address the dire economic situation. I hope in 2020 that Labour will tell the truth to the electorate and outline sensible policies to deal with the situation as it exists then and for the foreseeable future.
Now that Brexit has happened, it is highly likely that UKIP will decline. This means that their voters will now be up for grabs. Labour needs to re-connect with them. If they can, they will win; if not, they could lose.
What may make a difference is if Labour promotes hope over disillusion. Corbyn is an excellent person to achieve this; I cannot see anyone else on the scene with the ability to motivate people in the same way. His 10 Point programme released today should provide some flesh on the bones of policy which will win power for Labour and the people of Britain.