After returning from a 2 weeks holiday when I've left my 147 manual in 1st gear and handbrake (for good practice I'd think), now I wonder if I've made such mistake as now the gear lever becomes very sticky...

I can still engage all gears in working order but it is hard work and with some heavy resistance in particular the "horizontal" movements - i.e. from neutral to 1, 2-3, 4-5. It was all smooth and accurate enough prior to leaving it standing in gear.
From searching previous threads I gather this might be a common/easy fix for the 147? But what exactly has gone wrong? Is it just lubrication needed or something else has seized up or worn in the box? If I drive it would it make matter worse? I'm definitely not mechanically sound so would appreciate all comments/advice. Many thanks!