Essentially, if May loses just 5 seats and Labour holds or gains them, her Govt becomes illegitimate and she would have to seek either Parliament's approval to continue, or be forced into forming a coalition. As it stands, Labour and the SNP (the 2nd and 3rd biggest parties by MPs) will never go into coalition with the Tories and the only party that would will not have enough MPs to form the needed majority,
In that event, if Labour, SNP, Greens, the two bigger Irish Parties (both leftists) and the Welsh party Plaid Cymru agreed on forming a govt together they could then take over as a Left Wing multi-party coalition.